In motor vehicles, various processes are controlled by microprocessors. Typically, however, the microprocessors output signals at low levels. The devices to be controlled, however, sometimes require large currents or high voltages. A transformation of the low signal levels to the higher signal levels is achieved by final stages. Expediently, the final stages are situated in spatial proximity to the devices to be controlled. On the other hand, it proves expedient to situate the controlling microprocessors at one or a few central locations in the motor vehicle. The signals are expediently transmitted between the microprocessor and a final stage via a unidirectional data transmission link. Such a data transmission link is described e.g. in German Patent Application Nos. DE 19733748 and DE 10228905.
Various electrical interference fields exist in the vehicle, which may falsify the signal to be transmitted during the transmission. A mechanism for detecting a faulty transmission on the side of the final stage makes use of redundant data that are transmitted along with the signal. The final stage is then able to determine on the basis of the cross sum or parity of the transmitting signal whether a faulty transmission occurred or whether the integrity of the transmitting data block is preserved.
The data transmission between the microprocessor and the final stage occurs at a high data transmission rate. In the event of a faulty transmission it is not expedient to stop the transmission and to resend the faultily transmitted data block. On this matter, German Patent Application No. DE 19733748 provides for the transmission to be continued even in the event of a faulty transmission of an individual data block. In this case, the final stage is to retain the previously set output state until the next correct transmission of a data block occurs.
This method and the associated device result in faulty behavior in some of the devices to be controlled since high signal levels may be applied on them for too long.